The plugin has not been released yet. It is simple enough to copy JvmCommandPlugin.scala directly to your '~/.sbt/plugins/' folder.
See SBT Global Plugins.
Command is system, then use tab to list the available sub-commands.
Ex:
Setting a property
> system setProperty logback.debug true
[info] Setting logback.debug = true
Memory stats
> system memoryStats
[info] Memory
[info] total = 275.63 MB
[info] free = 134.19 MB
[info] used = 141.50 MB
[info] max = 1.33 GB
Published under the Apache License 2.0